Electrophoresis in slab-shaped gels is a common analytical technique in biotechnical laboratories, since a slab gel offers many beneficial features to the analytical chemist or biochemist. Included among these are the ability to divide the gel into several lanes and perform separate analyses in the various lanes under identical conditions, the ease of removing the gel from its enclosure and staining it to permit visualization of the bands, and the ease of scanning the gel and recording the results to identify and quantify the contents of each band. Slab gels are commonly held in cassettes, which are parallel plate enclosures that have a gap of precise width between the plates to define the gel thickness. The typical cassette is designed to permit the loading of samples along the top edge of the gel while exposing both the top and bottom edges of the gel to permit contact with upper and lower electrode buffers, respectively, when the cassette is placed in an electrophoresis cell. The gel is prepared in the cassette itself by filling the space between the plates with a solution of gel-forming monomer or prepolymer and allowing the solution to polymerize.
Casting the gel in this manner requires that the bottom edge of the gel space be temporarily sealed off in a manner that results in a gel with a cleanly defined bottom edge and yet allows the user to remove the seal without tearing or otherwise damaging the bottom edge of the gel. This is accomplished with a strip of flexible adhesive-backed tape. While the tape holds the gel liquid adequately and can be removed without tearing the gel, experience has shown that simply applying the tape across the two bottom edges of the two plates does not produce the most well defined gel boundaries. The two plates are welded together along their side edges, and the sealing tape must seal not only against the parallel bottom edges but also against the weld line which continues to the ends of the bottom edges. Also, the two bottom edges provide very little surface area for the tape to seal against.
These problems have been addressed in the prior art by designing the gel cassette so that the gel space makes a 90.degree. turn at the bottom. Thus, instead of a gel space opening at the very bottom of the cassette (with the two plates in a vertical orientation), the cassette has a gel space that passes through one of the two parallel plates and opens at the side of the cassette through a slot in one of the plates. No weld lines intersect with the slot, and the surface area surrounding the slot is planar and continuous since it is part of the broad face of the plate. Since the surrounding surface is considerably wider than the edge surfaces, this arrangement provides a greater surface area to which the tape can be sealed. Once the gel is cast and the electrophoretic separation has been completed, the two plates are broken away from each other along the slot to allow the gel to be lifted out for staining, drying or other electrophoretic processes. This is facilitated by using a wide hand tool in a stepped area along the two side edges and the bottom edge of the gel plates.
A gel formed in a cassette of this type therefore has a "foot" along its bottom edge, i.e., a lip projecting from the rest of the gel at a 90.degree. angle. The foot thus passes through a side opening or slot in the cassette that is coplanar with one of the plates and places the bottom edge of the gel flush with the outer surface of the plate. Once electrophoresis has been performed and the plates are split apart to expose the gel, the slot remains intact in one of the two pieces of the cassette, with the foot inside the slot. To extract the gel without damaging it, the foot must be pushed out through the slot. This is a delicate and difficult procedure that is prone to error and entails a risk of breaking the gel. In addition, once the gel is fully removed, the foot must be cut off before the gel can be dried, since the gel is dried in a flattened condition and the sharp right angle of the foot and the added thickness at the 90.degree. outer corner prevents the foot from being flattened. Pushing the gel through the slot and cutting off the foot both add to the time required to perform an electrophoretic analysis in a slab gel, and both provide opportunities for operator error.